Begonia pteridiformis was discovered in 2009 and has only been available in cultivation for a few years.  Native to Peninsular Thailand, it is found on wet limestone regions of lowland evergreen forest.   It is named for the unique arrangement of leaves that resemble a fern (Pteridi) at a quick glance.  Growing to only about 10″ tall, it makes an excellent Begonia for the terrarium, although I have also seen it grown in open-air successfully.  A tuberous species, which like many tuberous species, does not like it too wet, and should be allowed to dry out between waterings.  Despite its thin leaves, they are actually quite durable and can be used to propagate with a high success rate.  It might be best to turn the leaves upsidedown on a barely moist substrate for germination, as the leaves rot easily if kept too wet.  During dormancy, cease watering for a season and keep in lower light, and it will come back eventually. The bright red stems and dark, purple-green foliage contrast beautifully. The leaves are dramatically serrated with fine red hairs covering them.  It grows in a bush shape and can grow to be about two feet tall, but it would take a long time to get that large.  It is a fast-growing species once acclimated, and under the right growing conditions, but can also be finicky, prone to melting or dying off for reasons unknown to me.    My largest plants are about 8 inches but this species can get over a foot tall with time.

The last photo is representative of the plants available. They are well rooted and shipped meticulously in their pots.